1. Field of the Invention
This invention relates to communication systems and to optical and electronic signal processing elements that embodied in such communication systems.
2. State of the Art
Modern communication systems, such as phased-array communication systems, employ transmit and receive modules that are located nearby one another. In this configuration, the operation of the receive module may be interfered with by an overwhelming interference signal radiating from a nearby transmitter, thereby limiting the dynamic range of the system. For example, modern aircraft communication systems typically employ a number of radios which need to be operated simultaneously and in full duplex mode. Interference levels can be controlled by adjusting antenna spacing, frequency separation, transmitter power and specific signal delays. Such systems have been designed and built with operating frequencies from 2 MHz to 12.4 GHz with interference levels ranging from μwatts to watts.
Interference cancellation systems have been developed to further mitigate the effects of the interference signal radiating from the transmitter. A high level functional block diagram of an interference cancellation system (ICS) 10 is shown in FIG. 1. The transmitter 12 generates a transmit signal that is supplied to a transmitter antenna 14. The signal emanating from the transmitting antenna 14 is reduced in amplitude and delayed in time as it propagates away from the transmitting antenna. The reduction in amplitude is essentially proportional to 1/R2, where R is the distance between the point of observation and the transmitting antenna 14. The time delay is due to the finite velocity of propagation of electromagnetic fields, as is well known from the theory of retarded potential for electromagnetic fields. Suppose that s(t) denotes the transmit signal at any time t at the transmitting antenna 14. Then at the receiving antenna 16, the interference signal will be of the form Ks(t−τ), with K and τ being the amplitude reduction and the time delay, respectively. The medium between transmitter antenna 14 and receiver antenna 16 behaves as a network with a transfer function K/τ. The interference cancellation signal generator 18 is a synthesized network that is designed to have this same transfer function K/τ. A sample of the transmit signal s(t) is fed into the interference cancellation signal generator 18, which generates an interference cancellation signal that is identical to the propagating transmit signal received at the receive antenna 16. A summing stage 20 (typically realized by difference amplifier) subtracts the interference cancellation signal from the signal received at the receive antenna. The resultant signal produced by the summing stage 20, which is labeled the “desired receive signal”, is devoid of interference caused by the transmission at the nearby transmitter module (e.g., transmitter 12 and transmit antenna 14). This signal is supplied to the receiver 22 for subsequent signal processing (e.g., demodulation and baseband signal processing).
Ideally, the signal cancellation operations performed the ICS 10 should be completely independent of the characteristics of the transmit signal s(t), such as its amplitude, carrier frequency, type of modulation, degree of modulation, duty cycle and other characteristics.
A key design parameter for the ICS 10 is the selection of K and τ, whose values are never known but must be synthesized with a high degree of accuracy. If the transfer function characteristics (K and τ) of the interference cancellation signal generator 18 are not matched to the natural propagation path of the transmit signal s(t) at any time, there will be a difference of the signals and the output of the summing stage 20 will contain an error signal with the characteristics of transmit signal s(t). In the ICS 10 of FIG. 1, this error signal is cancelled out through a feedback loop provided by a control block 24. Control block 24 cancels out this error by adjusting the transfer function characteristics (amplitude reduction —K, and time delay —τ) of the interference cancellation signal generator 18 via control signals supplied to the interference cancellation signal generator 18.
Typically, the interference cancellation signal generator 18 of FIG. 1 utilizes electrical coaxial delays and electrical control circuits to provide the amplitude reduction and time delay that realize the desired transfer function K/τ. These approaches are limited in the sensitivity of the cancellation by the noise in the electronics and by the electromagnetic interference induced in the coaxial delay in a high power multiple system and multiple frequency environment.